The present invention relates to a frequency modulator constructing a wide band frequency demodulator such that a video signal is used as a modulation signal.
In a recent frequency demodulator which deals with a video signal, a system using a PLL (phase locked loop) comprising a phase comparator, a frequency modulator, and a loop amplifier is mainly used. The production of an IC for demodulators according to this system is currently conducted. In making the IC, it is a general practice that these circuits are constructed in a differential type thereby to perform signal processing such as amplification and the like.
The conventional frequency modulator will be described hereinbelow with reference to the drawings.
FIG. 1 shows a circuit of a conventional frequency modulator.
In FIG. 1, reference numeral 231 denotes a transistor; 232 indicates a transistor; 233 a constant current source; 234 and 235 resistors; 236 and 237 capacitors; 238 and 239 variable capacitance diodes; 240 a resonance coil; 241 and 242 base bias resistors; and 243 a power source. The transistors 231 and 232 are connected in a differential manner. The resistor 234 is connected between a collector of the transistor 231 and a voltage supply terminal C. The resistor 235 is connected between a collector of the transistor 232 and the voltage supply terminal C. The common emitters of the transistors 231 and 232 are connected to the constant current source 233. On the other hand, the capacitors 236 and 237 are connected in series and this series connection is inserted between the collector of the transistor 232 and a base of the transistor 231. A junction point of the capacitors 236 and 237 is connected to the ground through a tank circuit comprising the resonance coil 240 and the variable capacitance diodes 238 and 239 which are connected in series at their cathode sides. A control voltage terminal D is connected between cathodes of the diodes 238 and 239. The resistors 242 and 241 are connected in series between the bases of the transistors 232 and 231 in the order of the resistors 242 and 241. The power source 243 is applied to a node of the resistors 242 and 241.
The operation of the frequency modulator constructed as mentioned above will now be described hereinbelow.
First, a base bias of the differential amplifier composed of the transistors 231 and 232 is supplied by the power source 243 through the resistor 241 or 242. Each collector of the transistors 231 and 232 is connected to the voltage supply terminal C through the resistor 234 or 235. Each of the collector currents flowing through the transistors 231 and 232 is given by the constant current source 233, and a feedback loop is formed by the capacitor 236. A modulation signal is applied as a control voltage to the cathodes of the diodes 238 and 239 from the control voltage terminal D. Capacitances of the variable capacitance diodes 238 and 239 are determined by the control voltage given. A parallel resonance circuit, which is formed by the resonance coil 240 and the variable capacitance diodes 238 and 249, is connected to the base of the transistor 231 through the capacitor 237 and it resonates. An oscillating frequency is determined by a resonance frequency of the parallel resonance circuit and is outputted from each of the collectors of the transistors 231 and 232. E and F denote output terminals of the transistors 231 and 232.
However, in the above construction, the oscillating signal is applied to the base of one of the transistors, the base of the other transistor is connected to the ground, and the differential amplifier is used in the unbalanced state. Therefore, there is a problem such that a difference occurs between the oscillating outputs from the collectors of the first and second transistors.